In the EDM art, the discharge electrode is frequently mounted on an electrode holder which, in turn, is secured in a hydraulic chuck. The hydraulic chuck may be part of a mounting block, which secures the electrode holder. The electrode blank, affixed to the electrode holder, is then formed into the desired electrode shape. Once formed, the electrode holder may then be removed from the mounting block and then mounted in a similar hydraulic chuck in the EDM machine.
When using a hydraulic chuck to securely hold an electrode holder, both during the electrode forming step or the electrical discharge process, it is imperative that the electrode holder be prevented from moving. The application of proper clamping pressure is important because if insufficient clamping pressure is generated, the electrode holder has some play within the hydraulic chuck and thus adversely affects the system precision.
A system for generating a clamping pressure in a hydraulic chuck is described in West German Offenlegungsschrifts 26 46 951, 26 44 577, and 26 44 951. These systems comprise a set screw arrangement, wherein a set screw is accessible on the surface of the hydraulic chuck and communicates with a hydraulic piston via threads in a keeper nut. A hex key or other similar tool is inserted into the set screw and rotated a certain number of turns until the electrode holder is clamped. Generally, the operator remembers the number of turns required to clamp an electrode holder and employs the same method for each of the same diameter electrode holders.
There are several problems associated with this set screw pressure adjustment mechanism. First and foremost, the set screw system is time consuming and imprecise. In many cases, the electrode blank mounted on the electrode holder can be machined faster than the time required to adjust the set screw mechanism for the correct clamping pressure and thereafter releasing the set screw to remove the electrode holder.
Further, an operator must keep track of the number of times he or she has turned the hex key to develop the required clamping pressure. If the operator is distracted during this procedure and forgets how many times the hex key has been turned, the procedure must be restarted; otherwise, continued turning may result in an improper clamping pressure. Furthermore, it is often difficult to establish a known starting point for counting the number of turns the hex key should be rotated. Further yet, even if the operator has rotated the hex key the correct number of turns from an established starting point, there is little guarantee that the correct clamping pressure has been generated. In essence, this clamping procedure must rely on "feel" and is dependent on the experience level and care of the operator.
Another disadvantage of this system is the possibility of losing the hex key or other similar tightening tool. Often, the hex key falls into an oil-filled work tank and requires the operator to fish for the tool in the tank or drain the tank to find the tool.
Other patents related to this field are Ohsima, Hagemeyer et al., and Buck, U.S. Pat. Nos. 3,769,761, 4,533,287, and 3,731,942 respectively.